xDSL (Digital Subscriber Line) is a technique for high-speed data transmission through Unshielded Twist Pair (UTP). In addition to the base-band transmission DSLs, such as IDSL and SHDSL, the pass-band transmission xDSL utilizes the frequency division multiplexing (FDM) technique so that the xDSL service may coexist with the POTS (Plain Old Telephone Service) in a same twist pair. The xDSL service occupies the higher frequency band, while the POTS occupies the base band below 4 KHz. The POTS signal and the xDSL signal may be separated from each other by a splitter.
The pass-band transmission xDSL employs the discrete multi-tone (DMT) modulation. A system that provides multiple xDSL access is called a DSL access multiplexer (DSLAM), the system reference model of which is shown in FIG. 1.
It is well known that a higher line activation rate is generally desirable during the put-into-operation process of an xDSL service. To achieve a higher line activation rate, it is necessary to guarantee that the xDSL service be put into operation on more twist pairs of subscribers. In the actual application environment, however, xDSL service can not be put into operation normally on all the twist pairs of subscribers at an office.
Troubleshooting has to be carried out for the lines on which xDSL service can not be put into operation, or these lines have to be replaced completely, so as to enable the xDSL service to be put into operation. This process requires a great deal of manpower and material resources. As a result, the operating cost of the operators may be increased significantly.
In order to reduce the cost of troubleshooting, the Single-End Line test (SELT) technique is presented, that is, an automatic test means is employed to implement the testing, checking, and fault positioning of a line.
At present, the SELT test devices available in the market mainly employ TDR (Time Domain Reflection) method or Lumped Parameter Model estimation method for testing, in order to position a fault.
At present, a square wave, a sine wave, or a half-sine wave is usually used as the test signal in the existing TDR test devices. It can be seen that,
(1)Since the self-correlation of these waveforms is not strong, the receiving end can not distinguish correctly if there is a superimposition in the echo.
(2)If a test loop is long, the echo signal will be weak; accordingly, the signal is susceptible to interference of external signals, especially in the case of a square wave.
(3)When any of these waveforms is used in the test, the durations of test signal should be selected differently according to the lengths of the test loops. Thus, for the unknown lines, trial tests have to be carried out repeatedly, which will cost a long time.
(4)For a complicated test loop (e.g., when there are two bridge taps), those test signals will not function well sometimes.
(5)The line test may have adverse effects on the working xDSL lines in the same cable.